Thermal vs Electronic Overload Relays: Which to Choose

What is the difference between a thermal and electronic overload relay? A thermal (bimetallic) relay heats three bimetal strips with the motor current and trips a mechanical differential bar at a fixed setting ratio of roughly 1:1.5, almost always Class 10A per IEC 60947-4-1. An electronic (solid-state) relay reads current through current transformers or shunts and trips through a microcontroller model, with a wider 1:3 to 1:4 setting ratio and a selectable Class 5/10/20/30. That difference in construction decides what protection functions are on the table, how many relay sizes you need to stock, and what the relay costs installed. This article compares the two on setting range, trip class flexibility, phase-loss and ground-fault sensing, ambient behavior, and cost, then gives a straight answer on when each one is the right call.

How a Bimetallic Overload Relay Trips

Three bimetal strips, one per phase, carry the motor's line current directly. Heat from that current bends each strip; once the bend passes a mechanical set point, a differential trip bar releases the 95-96 NC contact and drops the contactor coil. The dial is calibrated in amps and set to the motor's nameplate full-load current (FLC). The trip curve is inverse-thermal (I²t): small overloads take minutes to trip, large ones seconds, which is shaped to survive a 6-8x FLC starting inrush without nuisance-tripping. No control power is needed — the relay runs entirely off the current it is protecting against. For the full construction breakdown see our guide on what a thermal overload relay is and how it works.

How an Electronic Overload Relay Trips

Current transformers or resistive shunts feed a microcontroller that runs a firmware thermal model of the motor instead of a physical bimetal strip. That model tracks accumulated heat the same way the winding does, including a thermal memory that survives a power cycle — a bimetal relay resets its thermal state the moment it cools, an electronic one remembers it was hot. The controller needs a low-voltage control supply, so the relay adds a wiring point a bimetal unit does not have. In exchange it gets a wider 1:3-1:4 setting ratio, a selectable trip class, and inputs for extra protection functions built into the same housing.

Setting Range and Trip Class: Where Electronic Pulls Ahead

A bimetal relay's narrow 1:1.5 ratio means a panel builder stocking motors from 5 A to 50 A FLC needs several relay frame sizes to cover the range. An electronic relay's 1:3-1:4 ratio lets one unit cover what took two or three bimetal relays, which cuts spare-parts SKUs on a large panel schedule. Trip class is fixed on most bimetal relays — Class 10A, tested at 7.2x the setting from cold — while electronic relays let you dial Class 10, 20, or 30 on the same unit. That matters when the same panel design has to serve both a standard pump (Class 10A) and a high-inertia fan (Class 20 or 30) without changing the relay part number. See our breakdown of overload relay trip classes 10A, 10, 20 and 30 for the full test definitions.

Protection Beyond Overload: Phase Loss, Imbalance, Stall

Both types can sense a lost phase, but they do it differently. A bimetal relay with a differential mechanism reacts to the uneven bending between phases: a lost phase forces roughly 1.7x current onto the remaining two windings, and the differential bar trips faster than the plain thermal element alone would. An electronic relay measures true phase-to-phase current imbalance as a percentage and can trip on imbalance alone, before any phase drops out completely, which catches a degrading connection earlier than a bimetal relay can. Electronic relays commonly add locked-rotor and stall detection — tripping fast if the motor draws high current without turning — a function no bimetal relay has, because a bimetal element cannot distinguish a stalled rotor from a normal 6-8x FLC start. Read more on phase-loss and single-phasing protection.

Ambient Sensitivity and Mounting

Bimetal relays include a compensating bimetal so the trip point does not drift much between roughly -5 and +55/60 C ambient, but the compensation is not perfect: a hot panel interior still shifts the effective trip point on a poorly-mounted unit, and the relay should sit in the same temperature zone as the motor's control gear. Electronic relays, sensing current through a CT rather than heating a physical strip, are markedly less sensitive to their own ambient temperature, which matters in enclosed panels with limited ventilation or in outdoor enclosures with wide daily temperature swings. Both types clip directly under their manufacturer's contactors — Schneider TeSys LRD under LC1D, ABB TA-series under A/AF, Siemens SIRIUS 3RU21 under 3RT2 — so mounting footprint is not a differentiator; thermal drift is.

Cost, Wiring and Where Bimetal Still Wins

A bimetal relay costs less per point and needs no control power wiring: it is a mechanical add-on to the motor circuit and nothing else. For a standard pump or fan running Class 10A duty with no ground-fault or stall requirement, that simplicity is the entire argument: fewer terminals, fewer failure points, lower installed cost. Electronic relays cost more per unit and add a control-supply terminal, but the extra spend buys back setting range, selectable trip class, and protection functions a bimetal relay cannot offer at any price. Reset behavior — hand or automatic — is a separate choice available on both types; see manual vs automatic reset on overload relays for when auto-reset is actually safe to use. What we see in the field: panel builders standardize on bimetal for the bulk of a schedule and reserve electronic relays for the handful of motors that actually need the extra range or protection, rather than specifying one type across the whole board.

Bimetallic vs Electronic Overload Relay: Comparison Table

Which One Should You Choose?

Start with the trip class the load actually needs. A standard pump or fan runs up in a few seconds and sits comfortably on Class 10A — a bimetal relay is enough, and cheaper. A large fan, centrifuge, or crusher with a run-up longer than a Class 10 curve tolerates needs Class 20 or 30, which points to electronic unless the manufacturer offers a fixed higher-class bimetal variant for that frame. Next check whether the application needs stall detection, ground-fault sensing, or thermal memory across power cycles — if any of those is a spec requirement, only electronic delivers it. Last, look at the FLC spread across the panel schedule: a wide spread favors electronic on part-count alone, even before protection functions are weighed. For the full construction and standards background behind both types, see the thermal overload relay engineering guide, and browse the current thermal overload relays range for both bimetal and electronic models across ABB, Schneider, and Siemens.

Conclusion

Bimetal and electronic overload relays protect the same fault — sustained motor overload above FLC — through different mechanisms, and that mechanism difference drives every practical distinction between them: setting range, trip class flexibility, phase-loss sensitivity, stall and ground-fault detection, ambient behavior, and cost. Match the relay type to the load's trip-class needs and any extra protection requirement first; default to bimetal for standard duty and reach for electronic when the panel schedule, load inertia, or spec sheet demands it.